[BUG][typescript-angular] Type mapping from DateTime to string causes build error

Created by: malymato

I encountered the same issue on v 4.1.3 as described here: https://github.com/OpenAPITools/openapi-generator/issues/2745

When using a type mapping from DateTime to string, generated code contains lines such as queryParameters = queryParameters.set('dateFrom', <any>dateFrom.toISOString());, which causes build failure.

This was fixed for typescript-axios generator, but the issue is still present in typescript-angular. I'd like to open a PR with a fix.